Whats new on the ground?Running Cherenkov gamma ray telescopesThmisDavid A. Smith Centre dtudes Nuclaires de Bordeaux-Gradignan,In2p3/CNRSCATCELESTELes Arcs23 January 2001

L'Astronomie Gamma

Very motivatingCosmic accelerators, probing theextragalactic medium, and all that: c.f. Sundays talks. A big problem~300 gamma sources at 1 GeV.Few well-studied sources at 200 GeV.Cherenkov imager sensitivity is good BUT the accelerators run out of gas, and absorption kicks in.

L'Astronomie Gamma

Example 1: Crab nebula and pulsar.Cornerstone of the Synchrotron+Inverse Compton paradigm, key to AGNs.Supernova remnants held to be source of high energy cosmic rays.Acceleration site in pulsars could be deduced from the energy of the spectral cutoff.But no other known steady source is as intense.(M. de Naurois thesis,astro-ph/0010264, 265,ApJ in preparation)Convenient number to remember:1 Crab = 10 -10 erg/cm2/s,around 100 GeV.

Cas A, IC443, g Cygni, CTB80

Pulsars very promising, see E. Durand talk.

L'Astronomie Gamma

Example 2: The blazar Mrk 501.Seen by Whipple before Egret : Cherenkov sensitivity is good.Blazars intrinsically very variable.Egret in the hole. Imagers on the IC bump, or beyond.Prediction from keV and radio results is that nearly no other blazars are so bright at 200 GeV.For 421 and 501, redshift z=0.03. Bigger z => X-galactic infrared absorption.Looking for things like 1ES 1101-232,1ES1959+65, 1ES 1426+528, 2EG J0222+4253,...

L'Astronomie Gamma

Markarian 421J. Buckley, Astropart. Ph. 11 p.119 (1999)Egret is out of the hole, and the sharp cut-off is beyond the imager range. EgretCAT, Whipple

L'Astronomie Gamma

The crying need in the field is to improve sensitivity and/or to lower the minimum energy threshold.This talkImager designs converging to an optimum:Big mirrors, fine cameras, and stereo.Solar farms are growing up.Milagro: a wide field-of-view without getting lost in space.Review of sources: see individual talks...

L'Astronomie Gamma

GlossaryImagers: Whipple (Arizona), CAT (Pyrenees), Hegra (Canary Islands),Cangaroo (Australia), Durham (Australia), Grace (India), Telescope array (Utah).Solar farms: Celeste (Pyrenees), Stacee (New Mexico), Solar-II (California), Graal (Spain).Milagro: Water Cherenkov, instead of atmospheric (New Mexico).

Future imagers: HESS array (Namibia), Veritas array (Arizona), Magic (Canary Islands), Cangaroo III (Australia)

Satellites: Egret on the Compton GRO, AGILE, gamma AMS, GLAST.

L'Astronomie Gamma

How to improve an imagerThresholds presently around 250 GeV, determined by Cherenkov signal to night sky noise ratio. Example 1: Whipple had biggest mirror (10 meters) and oldest camera. CATs mirror 5x smaller, but smallest pixel size (2 mr) and fastest electronics (few ns coincidence), for same threshold.Example 2: Cangaroo extended their 7 m mirror to 10 meter diameter.Energy threshold in 200 GeV range.

Sensitivity increases:finer cameras improve gamma/proton image separationstereo, for better alpha resolution for muon rejectionCAT Mrk 501 flare: A. Djannati et al, Astron. Astrophys. 350 (1999) 17-24.

L'Astronomie Gamma

Digitally combined composite of nine 8-minute exposures, November 18th 1999, 1h29-2h46 TU, Sharm El Sheihk, Egypt, by Nigel Evans, courtesy of Sky & Telescope, June 2000. All Leonid meteors radiate from a point just inside the sickleof Leo, whose bottom star, Regulus, is the brightest star at lower leftAlpha: perspective angle of parallel lines viewed from an offset position.Like, looking up at tall trees. Or looking at meteor paths in the sky.

L'Astronomie Gamma

A. Konopelko,Protons: fat, irregular images.Gammas: uniform, narrow.Alpha:Angle betweenellipse major axis and line fromimage center tocamera center.

L'Astronomie Gamma

A. Konopelko,

L'Astronomie Gamma

Remaining background in alpha plot is mainly muons. Rare event: muon arc together with hadron image. A muon passing through the imager mirror appears as a full circle, radius = Cherenkov angle.Circle center position gives muon direction relative to imager pointing direction.Muon at edge of mirror: half-circle.Muon distance D from mirror,arc length ~ 1/D.For CAT: few pixels at 8 meters.Can look just like low energy g.

For CAT: 20 Hz trigger rate,of which 12 Hz is muons.

Invisible beyond 12 meters.

Future big mirrors with lowthresholds => ~1 kHz muons/mirror.

Two mirror coincidence rejects muons.G. Vacanti et al, Astropart. Phys. 2 (1994) 1-11.

L'Astronomie Gamma

SUMMARY of developments from the principal imagers:Whipple now has a fine camera like CATCANGAROO now has a 10 meter mirror like WhippleCAT is studying poor mans stereo using CELESTE, to be a little bit like HEGRA (more on this later).

L'Astronomie Gamma

Sandia Laboratory, New Mexico: site of the STACEE experimentThere is more to life than just imagersOr: the choice of lower energy instead of higher sensitivity.

L'Astronomie Gamma

Wavefront sampling, revisitedAn imager measures the angular distribution of Cherenkov light at one place in the light pool (or multiple points, for stereo).

A different approach, wavefront sampling, was validated by ASGAT and Themistocle. The spatial and temporal light distributions are measured by mirrors at many points in the light pool. In the TeV range, imagers work best. BUT!Below 100 GeV, hadron showers produce little Cherenkov light. At g shower maximum, only Eg/2 electrons (15 electrons at 30 GeV), so statistical fluctuations and geomagnetic scattering dominate hadron/gamma differences. Wavefront samplers are blind to muons.

Below 100 GeV the advantages of imagers are diminished.

The geometry of a solar plant is a technical compromise (example: aberrations change as source is tracked) but allows fast, cheap access to 30 GeV.

L'Astronomie Gamma

Thmis(Pyrnes)CELESTE

CAT imager

ASGAT

Themistocle40 heliostats since 1999.Trigger threshold: 30 GeVAnalysis threshold: 50 GeV(at transit)13 heliostats being added.5 trigger groups

L'Astronomie Gamma

30 ton boiler removed from tower, replaced by spherical secondary mirrors.Winston cones for sharp field-of-view (10 mr).Nanosecond phototubes & electronics optimize the cherenkov signal vs. night sky light noise.Programmable delays track celestial rotation.One of six camerasData acquisition based on 1 GHz Flash ADCs.

( See talk by E. Durand )

L'Astronomie Gamma

Trigger rate versus thresholdAccidental coincidencesfrom night sky lightCherenkovSimulation for 4 p.e. per heliostat threshold, assuming Crab spectrum, at transit.Trigger: 5 analog sums, 8 heliostats each.3 of 5 logic coincidence.Monte Carlodata

L'Astronomie Gamma

( See Heidelberg proceedings, astro-ph/0010264, 265 )Padding adapted to Flash ADC data, to decrease sensitivity to background light.

L'Astronomie Gamma

( A hadron veto scheme being tested: further increase? )( Whipples sensitivity in 1991 )

L'Astronomie Gamma

Poor mans stereo: CAT and Celeste record same showers, provides muon rejection for CAT.Improve performance of both telescopes, study spectra from 30 GeV to many TeV.

L'Astronomie Gamma

L'Astronomie Gamma

Mrk421 by Celeste.

(No detection of 501, 1ES0219+428, 1ES2344+514.IC443 most promising supernova remnant.Pulsed study of Crab and PSR1951+32, see E. Durand talk).

L'Astronomie Gamma

Four solar farms are on track. Besides Celeste,STACEE (Sandia, New Mexico)Crab detection at 190 GeV with preliminary detector (S. Oser et al, Ap. J. in press).Currently, shakedown of upgraded instrument, expect 50 GeV threshold.Solar-II (Barstow, California)First light: have tracked the Crab while recording air showers.See talk by G. Mohanty.GRAAL (Almeira, Spain)No secondary optics. Instead, big phototubes mix light from several heliostats, for high energy threshold.See talk by M. Diaz Trigo.

L'Astronomie Gamma

Solar-II 2000 heliostats available (10x Celeste or Stacee)

L'Astronomie Gamma

Milagro can provide sorely needed alerts for Cherenkov telescopes.Pointing instruments (e.g. air Cherenkov) generally have better sensitivity than survey instruments. Counter example: Glast.

Narrow field-of-view problems: i) you necessarily have an a priori discovery goal - serendipity almost ruled out.ii) While looking at one sleeping blazar, another can flare behind your back.

Cherenkov telescopes watch X-ray satellites, optical monitors, and each other for alerts.

Help from Milagro ?( See talk by Jordan Goodman )

L'Astronomie Gamma

CONCLUSIONS.After a great start (Crab, 421, 501), ground-based gamma-ray astronomy turns out to be a tougher business than some of us thought.We are getting more clever about which blazars & SNRs might be the best bets (dont miss Thursdays talks!)While waiting for the next generation of instruments, the imagers are converging towards similar fundamental design choices.(improved sensitivity through hadron image rejection, muon rejection, and alpha resolution) solar farms have gotten their first results, with improvements in progress.(More sources available at lower energy, even if sensitivity is modest at first) Southern sky coverage constantly improving Milagro brings the advantages of air shower arrays (24 hour northern sky coverage) down to the atmospheric Cherenkov energy range (>200 GeV). The future imagers will go under 100 GeV with high sensitivity, from 2003.

L'Astronomie Gamma